// Copyright (c) 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// This file defines some bit utilities.

#ifndef BASE_BITS_H_
#define BASE_BITS_H_

#include <stddef.h>
#include <stdint.h>

#include "base/compiler_specific.h"
#include "base/logging.h"

#if defined(COMPILER_MSVC)
#include <intrin.h>
#endif

namespace base {
namespace bits {

    // Returns the integer i such as 2^i <= n < 2^(i+1)
    inline int Log2Floor(uint32_t n)
    {
        if (n == 0)
            return -1;
        int log = 0;
        uint32_t value = n;
        for (int i = 4; i >= 0; --i) {
            int shift = (1 << i);
            uint32_t x = value >> shift;
            if (x != 0) {
                value = x;
                log += shift;
            }
        }
        DCHECK_EQ(value, 1u);
        return log;
    }

    // Returns the integer i such as 2^(i-1) < n <= 2^i
    inline int Log2Ceiling(uint32_t n)
    {
        if (n == 0) {
            return -1;
        } else {
            // Log2Floor returns -1 for 0, so the following works correctly for n=1.
            return 1 + Log2Floor(n - 1);
        }
    }

    // Round up |size| to a multiple of alignment, which must be a power of two.
    inline size_t Align(size_t size, size_t alignment)
    {
        DCHECK_EQ(alignment & (alignment - 1), 0u);
        return (size + alignment - 1) & ~(alignment - 1);
    }

// These functions count the number of leading zeros in a binary value, starting
// with the most significant bit. C does not have an operator to do this, but
// fortunately the various compilers have built-ins that map to fast underlying
// processor instructions.
#if defined(COMPILER_MSVC)

    ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x)
    {
        unsigned long index;
        return LIKELY(_BitScanReverse(&index, x)) ? (31 - index) : 32;
    }

#if defined(ARCH_CPU_64_BITS)

    // MSVC only supplies _BitScanForward64 when building for a 64-bit target.
    ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x)
    {
        unsigned long index;
        return LIKELY(_BitScanReverse64(&index, x)) ? (63 - index) : 64;
    }

#endif

#elif defined(COMPILER_GCC)

    // This is very annoying. __builtin_clz has undefined behaviour for an input of
    // 0, even though there's clearly a return value that makes sense, and even
    // though some processor clz instructions have defined behaviour for 0. We could
    // drop to raw __asm__ to do better, but we'll avoid doing that unless we see
    // proof that we need to.
    ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x)
    {
        return LIKELY(x) ? __builtin_clz(x) : 32;
    }

    ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x)
    {
        return LIKELY(x) ? __builtin_clzll(x) : 64;
    }

#endif

#if defined(ARCH_CPU_64_BITS)

    ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x)
    {
        return CountLeadingZeroBits64(x);
    }

#else

    ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x)
    {
        return CountLeadingZeroBits32(x);
    }

#endif

} // namespace bits
} // namespace base

#endif // BASE_BITS_H_
